1. Field of the Invention
The present invention relates to a novel material applicable to a light-emitting element at least part of which includes an organic compound, and also relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device using the material.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer which includes a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance having a light-emitting property.
Since such a light-emitting element is of self-light-emitting type, it is thought that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. Besides, such a light-emitting element has advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained by formation of an element having a large area. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to lighting and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In an organic EL element using an organic compound as a light-emitting substance, by voltage application to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer including the light-emitting organic compound, and a current flows. Then, both the electrons and holes recombine to form an excited state in the light-emitting organic compound, and the excited state returns to a ground state, whereby luminescence occurs.
Having such a mechanism, the above-described light-emitting element is called a current-excitation light-emitting element. Note that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state. Luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In addition to light emission by recombination of carriers excited with a current, there is also a method of light emission in which excitation energy is transferred from an organic compound excited with a current to another organic compound and accordingly the latter organic compound is excited to emit light. This method is effective against an emission efficiency reduction (concentration quenching) due to stacking interaction caused by a high concentration of organic molecules that are desired to emit light. In organic EL elements, the method is generally applied to the element structure in which a light-emitting material is dispersed in a light-emitting layer (a light-emitting layer is doped with a light-emitting material). Doping a host material with organic molecules that are desired to emit light suppresses the stacking interaction, whereby efficiency of a light-emitting element can be increased. In such a light-emitting element, excitation energy is transferred from a host material excited by current excitation to a dopant material, making the dopant material emit light. Note that when Substance A is dispersed in a matrix formed of Substance B, Substance B forming the matrix is called a host material while Substance A dispersed in the matrix is called a dopant material.
Light emitted from a light-emitting material is peculiar to the material. It is very difficult to obtain light-emitting elements that emit light of good color and to fulfill important conditions such as lifetime and power consumption. The significant performances on lifetime, power consumption, and the like of light-emitting elements depend not only on substances that emit light but also largely on layers other than a light-emitting layer, element structures, compatibility between a light-emitting substance and a host, etc. Therefore, materials having various molecular structures have been proposed as light-emitting element materials (e.g., see Patent Document 1).
Further, commercialization of light-emitting elements makes a lifetime increase an important issue. In addition, light-emitting elements have been expected to exhibit improved properties.